The present invention relates to gamma camera scintillation plate assemblies and methods which may be used to detect both high-energy and low-energy radiation emissions for PET and SPECT imaging.
Crystal scintillation detector plates are widely used in nuclear medicine to image tissues containing radioactive tracer compounds (radiopharmaceuticals). introduced into a patient. Gamma quanta such as photons from the radioactive tracer compounds are collimated into the detector plate. The photons interact with a scintillation crystal or crystals in the detector plate to produce photons of light-scintillations.
A matrix of photomultiplier tubes is optically coupled to the detector plate""s window to detect the scintillations produced in the crystal. The camera""s electronics analyze the relative pulse heights from the photomultiplier tubes to compute the locations of the scintillation events in the crystal and to evaluate the corresponding intake of the tracer by the tissue. This information is expressed in an image.
Several different nuclear medicine cameras have evolved for particular applications. One type of camera utilizes single-photon emission computed tomography (SPECT) imaging. These cameras are typically used with radiopharmaceuticals such as thallium and technetium that produce lower energy (approximately 140 keV) gamma rays. These cameras generally have detectors with large, thin scintillating crystals. SPECT cameras often use a collimator that acts as a lens to create the image slices that are assembled by the computer into an image. Such cameras generally produce good resolution at a reasonable cost for some types of studies.
Another type of camera utilizes positron emission tomography (PET) imaging. Typical PET systems use fluorine-18 deoxyglucose (FDG) as a radiopharmaceutical. As FDG decays it emits two 511 keV gamma rays resulting from electron-positron annihilation. PET imaging systems generally have thicker crystals to stop the high-energy radiation before it passes through the crystal.
In both the SPECT and the PET systems, choosing a crystal thickness involves a tradeoff between the desire to convert a significant amount of radiation to light (which makes a thicker crystal more desirable) and the desire to achieve high image resolution (which makes a thinner crystal more desirable).
Gamma camera manufacturers have created hybrid cameras with dual detector heads, one of which is designed to operate in a SPECT mode and the other of which is designed to operate in PET mode. However, such hybrid systems are significantly more costly than SPECT systems, and suffer from lower sensitivity as compared to PET scanners.
From the foregoing, it will be understood that a need exists for improved crystal scintillation detector plates which may be used for both PET imaging and SPECT imaging.
The present invention provides a gamma camera scintillation crystal plate assembly and method that may be used for both PET (positron emission tomography) and SPECT (single photon emission computer tomography). The crystal plate assembly includes two or more crystals which are optically coupled. The result is one or more internal interfaces at which scintillation light is reflected, refracted, or scattered to prevent the loss of resolution that otherwise would arise in a crystal plate formed by a single crystal of the same thickness. The two or more crystals may be made of the same material or may be made of different materials. The crystal materials, the surface treatments of the crystals (such as their roughness), and the characteristics of the scattering at the internal interface(s) may be selected so as to tune the output of the gamma camera, such as by combining together the photopeaks of the two or more crystals, or alternatively by separating the photopeaks of the crystals.
According to one aspect of the invention, a crystal plate assembly for a gamma camera comprises an entrance scintillation crystal having an entrance surface and a coupling surface, and a downstream scintillation crystal having an interface surface and an exit surface. The interface surface is optically coupled to the coupling surface, and at least one of the two surfaces of one of the crystals is rougher than said surfaces of the other crystal, whereby the relative light output of the crystals is controlled.
In one preferred embodiment, the amount of scintillation light passing through the exit surface of the downstream crystal due to and in the vicinity of a scintillation event in the entrance crystal is substantially the same as an amount of scintillation light passing through the exit surface due to and in the vicinity of a like scintillation event in the downstream crystal. In an alternative preferred embodiment, the amount of scintillation light passing through the exit surface due to and in the vicinity of a scintillation event in the entrance crystal is sufficiently different from an amount of scintillation light passing through the exit surface due to and in the vicinity of a like scintillation event in the downstream crystal, such that the energy peak of the former falls in an energy valley of the latter.
In a preferred embodiment, the entrance crystal includes at least one of said surfaces that is rougher than said surfaces of the other crystal.
In a preferred embodiment, at least one of said surfaces that is rougher than said surfaces of the other crystal is the entrance surface of said entrance crystal.
In a preferred embodiment, the downstream crystal includes said at least one of said surfaces that is rougher than said surfaces of the other crystal.
In a preferred embodiment, the crystals are made of the same material.
In another preferred embodiment, the crystals are made of different materials.
In a preferred embodiment, the material of the entrance crystal generates more scintillation light in response to incident high energy radiation than the material of the downstream crystal in response to like incident high energy radiation.
In a preferred embodiment, an optical coupling material optically couples the interface surface and the coupling surface.
In a preferred embodiment, the entrance crystal material is hygroscopic and the downstream crystal material is non-hygroscopic.
In a preferred embodiment, the entrance crystal material is NaI(TI).
In a preferred embodiment, the downstream crystal material is BGO.
In a preferred embodiment, the entrance crystal and the downstream crystal have a combined thickness of at least 0.75 inches.
In another preferred embodiment, the entrance crystal and the downstream crystal have a combined thickness of at least one inch.
In a preferred embodiment, the coupling surface is rougher than the. interface surface.
In a preferred embodiment, the downstream crystal has a light absorbing material at at least one side surface thereof.
According to another aspect of the invention, a detector plate assembly for a gamma camera comprises a crystal plate assembly including an entrance scintillation crystal and a downstream scintillation crystal optically coupled to the entrance scintillation crystal. The crystal plate assembly has an exit surface for optical coupling to one or more light sensing devices, and the crystals have been tuned either (i) to match at the exit surface the energy of the photopeaks of light originating from the respective crystals or (ii) to locate the photopeak of one in the energy valley of the other.
In a preferred embodiment, the crystals have been tuned by imparting different surface finishes to surfaces of the crystals.
According to another aspect of the invention, a method of making a crystal plate assembly for a gamma camera comprises tuning two or more scintillation crystals to be optically coupled in a crystal plate assembly so as to control the energy position of the photopeaks at an output surface of the plate assembly; and optically coupling the crystals.
In a preferred embodiment, the tuning includes tuning the crystals such that photopeaks of at least two of the crystals are separated at the output surface of the plate assembly.
In another preferred embodiment, the tuning includes tuning the crystals such that photopeaks of at least two of the crystals are brought together at the output surface of the plate assembly.
In a preferred embodiment, the tuning includes selecting different materials for each of the two or more crystals. Preferably, one of the materials generates more scintillation light in response to incident radiation than another of the materials to like radiation.
In a preferred embodiment, the tuning includes treating surfaces of the crystals to vary the amount of internal reflection and scattering at the surfaces of the crystals.
In a preferred embodiment, the two or more crystals include an entrance crystal and a downstream crystal, the tuning includes providing a relatively less rough surface on the downstream crystal and a relatively rougher surface on the entrance crystal, and the optically coupling includes optically coupling the relatively less rough surface and the relatively rougher surface.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.